1. FIELD OF THE INVENTION
The present invention relates to an optical fiber measuring system using an interferometer.
2. SUMMARY OF THE PRIOR ART
It is known to supply radiation, from e.g. a laser diode, to an interferometer and then, by monitoring by any changes in the output of the interferometer, to detect changes in that interferometer. These latter changes may be caused, for example, by temperature changes or vibrations which alter parameters of the interferometer, and by monitoring the changes in the output the changes in the interferometer may be determined and so determine the temperature change involved.
When radiation is passed through a single fiber optic interferometer system, its intensity is modulated in proportion to the cosine of the optical path inbalance of the system. Methods are known for detecting and measuring accurately the phase of this cosine function. Provided that the coherence length of the radiation source is greater than the optical path length of the interferometer, such cosine modulation will be reduced. Furthermore, the change in phase of the interferometer output is inversely proportional to the wavelength used and directly proportional to the change in optical path difference.
However, a sensor utilising this principle gives unambiguous results over only one interferometer period. Since this is related to, and is of the order of, the wavelength of radiation employed the dynamic range of sensor is severely limited, particularly when optical wavelengths are used. Any larger change results in a change in an unknown number of integral interferometer periods, in addition to the fraction of the period which can be detected (it is only possible from phase relationship to determine fractions of a period). It would be possible to provide a system in which the number of integral period changes was counted, and then the fraction of a period determined from the phase, but this method is both difficult to achieve and depends on having some suitable reference from which the count may be started. Thus, it would be possible to calibrate such a system to a known temperature, and then detect the changes from that temperature by counting integral period changes and then determining the additional fractional change, but clearly the system is dependent on the accuracy of the reference, and also the information about that reference is lost when the sensor is turned off, whether deliberately or due to accidental power failure.